The present invention relates to overfill prevention assemblies for storage tanks and more particularly to such assemblies for above ground storage tanks.
Above ground liquid storage tanks for fuel, oil, chemicals, and other liquids conventionally include a containment apparatus located at the tank top to enable access and external hose connection to a fill adaptor. A vertical fill pipe extends downward with its lower end spaced above the tank interior bottom to deliver liquid at that location. A liquid level sensing shut-off valve assembly positioned inside the tank couples the fill adaptor to the top of the fill pipe.
Functions attempted to be performed by conventional shut-off valve assemblies include cutting of the delivery flow when the storage tank is nearly full and bleeding into the tank the fuel remaining in the adaptor and delivery hose after the delivery truck operator closes the valves on the truck. To accomplish this, a float member located outside the assembly rises as the liquid level approaches the top of the chamber. When the float reaches a predetermined upward position, the float causes a shut-off valve to enter the path of the delivery fuel stream. The force of the stream closes the shut-off valve against a valve seat thus sealing closed the main delivery opening in the assembly. Additional valves and ports may be provided to bleed the fuel remaining in or thereafter entering the fill adaptor either into the storage tank chamber or through a bypass channel to the lower part of the fill pipe.
Many problems arise from the presence of these shut-off valve sections in above ground tanks. For example, the valves and their many associated moving springs, cams, and pins break or become defective or worn due to the repeated slamming of the valve.
These problems could lead to the valve remaining in its closed position even when the float falls to its lowest position. In this case, the shut-off assembly must be pulled from the tank for repair.